Octahedral carbon coordination

The carbon atoms of most binary metal carbides have hypercoordinated environments like those shown in Figure 1.5. In particular, octahedral carbon coordination is common in the interstitial carbides formed by many transition metals, materials of variable composition in which carbon atoms... 

In the binuclear complex, [(bpy)2(0H2)Zn(p2-C03)Zn(bpy)2] (N03)2-7H20 (F), the two N4-bound Znx and Zn2 ions are bridged by carbonate in monodentate and bidentate modes, the Znx being coordinated to one H20. Both the zinc ions are octahedrally coordinated with a relatively greater degree of distortion at Zn2 due to the chelating mode of the carbonate bridge. In these binuclear carbonate complexes of Zn(II) and Cu(II) an unexpected behavior, viz. Zn-0 distances < distances for carbonate coordination, was reported (341). 

As the above discussion shows, we will adopt the simple view that an octahedrally hexa-coordinated metal complex, ML6, is coordinatively saturated, just as is a tetracoordinated carbon atom. Metal complexes with fewer than six ligands will be treated on the same footing as organic reactive intermediates. The distinction we are making is that direct involvement of the metal center in a reactive process (bond making or breaking) is easy only at coordinatively unsaturated metal centers, just as it is at carbon. Of course, just as at saturated carbon, ligand substitution by association or dissociation can take place, and the presence of the center (C or M) with its array of substituents can influence... 

Carbon monoxide binds weakly as a cr-donor ligand to four-coordinate co-balt(II) systems. Despite a bout of artifactual excitement, CO has never been observed to bind significantly to five-coordinate Co systems with a nitrogenous axial base to yield octahedral six-coordinate species. - The sulfur analogue thiocarbonyl (CS), although not stable as a free entity, binds very strongly to iron-porphyrin species in a linear manner. ... 

Six-coordination is obtained in [PtMe3(acac)]2 by bidentate (0,0 ) behaviour and by a bond to the 7-carbon, a situation maintained in solution at room temperature (on warming, the bond to the 7-carbon breaks). In the bipyridyl adduct, it is the bond to the 7-carbon that completes the octahedral coordination [192],... 

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